I'm still chomping at the bit for the XNA Framework and XNA Studio, which will allow .NET developers to write code that targets both Windows and the Xbox 360. Here are some XNA Studio resources: the XNA Developer Center, XNA FAQ, XNA Game Studio Express Forum, XNA Framework Forum, XNA Team Blog, Mitch Walker's Blog, and Tom Miller's Blog.
The Runtime: .NET Compact Framework 2.0
The XNA Framework makes use of a custom, native implementation of the .NET Compact Framework 2.0 Common Language Runtime (CLR) on the Xbox 360. Developers use the Compact Framework runtime today to develop applications for Windows Mobile devices, such as PocketPCs and Smartphones. Please see the MSDN site for more information about the Compact Framework.
Windows Mobile 5.0 Developer Resource Kit
The Windows Mobile 5.0 Developer Resource Kit contains all of the professional tools, technologies and technical resources you need to create innovative applications for Windows Mobile (including the .NET Compact Framework). Members of MSDN Connection got it for free. If you'd like to receive a copy yourself, just sign up to MSDN Connection Ireland. Instructions for how to obtain the resource kit are on Clare's blog.
A common architecture for sometimes-connected systems sees multiple subsets of a central database synchronized with multiple devices. (Think of applications like field-force automation, where a mobile workforce wants rapid access to parts of a larger database that they occasionally update.)
If you are developing for that sort of scenario for the Compact Framework, take a look at SQL Server 2005 Everywhere Edition - a lightweight, flexible, free datastore that is ideal for that sort of scenario. Here is the SQL Server Everywhere website (see the handy comparison chart that compares SQL Server Express to SQL Server Everywhere), the datasheet (with details, in Word format), a link to the Community Tech Preview download (with a release date target of the end of 2006). (I previously wrote about it here.)
[Update] Mobile Line of Business Solution Accelerator Kit
Hot off the presses is this resource from Rob Tiffany that helps developers understand the challenges and architecture that must be taken into consideration when building Windows Mobile solutions for Line of Business (LOB) applications. Providing secure and available access to corporate data via a platform that is occasionally disconnected can be a daunting task for developers who are new to the mobile world. This Accelerator kit includes a fully functional Supply Chain application including full source code (with over 6,000 lines of commented code) that demonstrates the best practices. Download the Accelerator kit here.